Re: [dpdk-dev] [PATCH] vhost: batch used descriptors chains write-back with packed ring

[Maxime Coquelin]

Hi Ilya,

On 12/12/18 4:23 PM, Ilya Maximets wrote:

On 12.12.2018 11:24, Maxime Coquelin wrote:

Instead of writing back descriptors chains in order, let's
write the first chain flags last in order to improve batching.

With Kernel's pktgen benchmark, ~3% performance gain is measured.

Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
---
  lib/librte_vhost/virtio_net.c | 39 +++++++++++++++++++++--------------
  1 file changed, 24 insertions(+), 15 deletions(-)

Hi.
I made some rough testing on my ARMv8 system with this patch and v1 of it.
Here is the performance difference with current master:
     v1: +1.1 %
     v2: -3.6 %

So, write barriers are quiet heavy in practice.

Thanks for testing it on ARM. Indeed, SMP WMB is heavier on ARM.

To reduce the number of WMBs, I propose to revert back to original
implementation by first writing all .len and .id fields, do the write
barrier then writing all flags by writing first one last:

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 5e1a1a727..58a277c53 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -136,6 +136,8 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
 {
        int i;
        uint16_t used_idx = vq->last_used_idx;
+       uint16_t head_idx = vq->last_used_idx;
+       uint16_t head_flags = 0;

        /* Split loop in two to save memory barriers */
        for (i = 0; i < vq->shadow_used_idx; i++) {
@@ -165,12 +167,17 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
                        flags &= ~VRING_DESC_F_AVAIL;
                }

-               vq->desc_packed[vq->last_used_idx].flags = flags;
+               if (i > 0) {
+                       vq->desc_packed[vq->last_used_idx].flags = flags;

-               vhost_log_cache_used_vring(dev, vq,
+                       vhost_log_cache_used_vring(dev, vq,
                                        vq->last_used_idx *
                                        sizeof(struct vring_packed_desc),
                                        sizeof(struct vring_packed_desc));
+               } else {
+                       head_idx = vq->last_used_idx;
+                       head_flags = flags;
+               }

                vq->last_used_idx += vq->shadow_used_packed[i].count;
                if (vq->last_used_idx >= vq->size) {
@@ -179,7 +186,13 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
                }
        }

-       rte_smp_wmb();
+       vq->desc_packed[head_idx].flags = head_flags;
+
+       vhost_log_cache_used_vring(dev, vq,
+                               vq->last_used_idx *
+                               sizeof(struct vring_packed_desc),
+                               sizeof(struct vring_packed_desc));
+
        vq->shadow_used_idx = 0;
        vhost_log_cache_sync(dev, vq);
 }

My testcase is the three instances of testpmd on a same host (with v11 from 
Jens):

     txonly (virtio_user0) --> fwd mode io (vhost0, vhost1) --> rxonly 
(virtio_user1)

Best regards, Ilya Maximets.

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 5e1a1a727..c0b3d1137 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -135,19 +135,10 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
                        struct vhost_virtqueue *vq)
  {
        int i;
-       uint16_t used_idx = vq->last_used_idx;
+       uint16_t head_flags, head_idx = vq->last_used_idx;

- /* Split loop in two to save memory barriers */

-       for (i = 0; i < vq->shadow_used_idx; i++) {
-               vq->desc_packed[used_idx].id = vq->shadow_used_packed[i].id;
-               vq->desc_packed[used_idx].len = vq->shadow_used_packed[i].len;
-
-               used_idx += vq->shadow_used_packed[i].count;
-               if (used_idx >= vq->size)
-                       used_idx -= vq->size;
-       }
-
-       rte_smp_wmb();
+       if (unlikely(vq->shadow_used_idx == 0))
+               return;

for (i = 0; i < vq->shadow_used_idx; i++) {

                uint16_t flags;
@@ -165,12 +156,24 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
                        flags &= ~VRING_DESC_F_AVAIL;
                }

- vq->desc_packed[vq->last_used_idx].flags = flags;

+               vq->desc_packed[vq->last_used_idx].id =
+                       vq->shadow_used_packed[i].id;
+               vq->desc_packed[vq->last_used_idx].len =
+                       vq->shadow_used_packed[i].len;
+
+               rte_smp_wmb();

- vhost_log_cache_used_vring(dev, vq,

+               if (i > 0) {
+                       vq->desc_packed[vq->last_used_idx].flags = flags;
+
+                       vhost_log_cache_used_vring(dev, vq,
                                        vq->last_used_idx *
                                        sizeof(struct vring_packed_desc),
                                        sizeof(struct vring_packed_desc));
+               } else {
+                       head_idx = vq->last_used_idx;
+                       head_flags = flags;
+               }

vq->last_used_idx += vq->shadow_used_packed[i].count;

                if (vq->last_used_idx >= vq->size) {
@@ -179,8 +182,14 @@ flush_shadow_used_ring_packed(struct virtio_net *dev,
                }
        }

- rte_smp_wmb();

+       vq->desc_packed[head_idx].flags = head_flags;
        vq->shadow_used_idx = 0;
+
+       vhost_log_cache_used_vring(dev, vq,
+                               head_idx *
+                               sizeof(struct vring_packed_desc),
+                               sizeof(struct vring_packed_desc));
+
        vhost_log_cache_sync(dev, vq);
  }